How Do Packets Make Their Way Through Packet-Switched Networks

Earlier we said that a router takes a packet arriving on one of its connected communication links and forwards that packet on to another of its connected communication links. But how does the router decide the link onto which it should forward the packet? This is actually done in different ways by different types of computer networks. In this introductory section, we will explain one popular approach, namely, the approach employed by the Internet

In the Internet, each packet traversing the network includes the address of the packet's destination in its header. As with postal addresses, this address has a hierarchical structure. When a packet arrives at a router in the network, the router examines a portion of the packet’s destination address and forwards the packet to an adjacent router. More particularly, each router has a forwarding table that maps destination addresses (or portions of the destination addresses) to outbound links. When a packet arrives at a router, the router examines the address and searches its table using this destination address to find the proper outbound link. The router then directs the packet to this outbound link.

We just learned that a router uses a packet's destination address to index a forwarding table and decide the proper outbound link. But this statement begs yet another question: how do forwarding tables get set? Are they configured by hand in each and every router, or does the Internet use a more automated procedure? This issue will be studied in depth in "The Network Layer". But to stimulate your appetite here, we’ll note now that the Internet has a number of special routing protocols that are used to automatically set the forwarding tables. A routing protocol may, for instance, decide the shortest path from each router to each destination and use the shortest path results to configure the forwarding tables in the routers.

The end-to-end routing process is similar to a car driver who does not use maps but instead prefers to ask for directions. For instance, assume Joe is driving from Philadelphia to 156 Lakeside Drive in Orlando, Florida, Joe first drives to his neighborhood gas station and asks how to get to 156 Lakeside Drive in Orlando, Florida. The gas station attendant extracts the Florida portion of the address and tells Joe that he needs to get onto the interstate highway I-95 South, which has an entrance just next to the gas station. He also tells Joe that once he enters Florida he should ask someone else there. Joe then takes I-95 South until he gets to Jacksonville, Florida, at which point he asks another gas station attendant for directions. The attendant extracts the Orlando portion of the address and tells Joe that he should continue on I-95 to Daytona Beach and then ask someone else. In Daytona Beach another gas station attendant also extracts the Orlando portion of the address and tells Joe that he should take I-4 directly to Orlando. Joe takes I-4 and gets off at the Orlando exit. Joe goes to another gas station attendant and this time the attendant extracts the Lakeside Drive portion of the address and tells Joe the road he must follow to get to Lakeside Drive. Once Joe reaches Lakeside Drive, he asks a kid on a bicycle how to get to his destination. The kid extracts the 156 portion of the address and points to the house. Joe finally reaches his ultimate destination.

In the above analogy, the gas-station attendants and kids on bicycles are equivalent to routers. Their forwarding tables, which are in their brains, have been configured by years, of experience,

How would you actually like to see the end-to-end route that packets take in the Internet? We now invite you to get your hands dirty by interacting with the Traceroute program, by visiting the site http://www.traceroute.org. (For a discussion of Traceroute, see "Delay, Loss and Throughput in Packet-Switched Networks")

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